Due to recent advances in the technology of laser generation and detection, laser systems for use in battlefield conditions have become more and more prevalent. These laser systems are employed for target illumination and tracking or for ranging. Such laser systems may also be employed for intentional blinding of personnel or sensors. In a particular battlefield setting, there may be numerous laser illuminators operating simultaneously. These laser illuminators may be both from friendly forces and from enemy forces. Combat troops operating in this environment will be subject to uncontrolled illumination by laser radiation. Because of the great radiated power from these laser radiation sources, these personnel require some eye protection from this laser illumination.
There have heretofore been suggested numerous solutions for this problem of laser protection. In particular, in recent years there has been an increased interest in the development of modulated index of refraction filter elements as laser protection devices. Modulated index of refraction filter elements include 3-dimensional modulated index of refraction patterns which diffract light at specified wavelengths. Such modulated index of refraction filter elements may be constructed of multilayer dielectric filters or of holographic optical elements. Multilayer dielectric filters are typically constructed from transparent layers of differing indices of refraction vapor deposited on a substrate. Holographic optical elements are ordinarily constructed employing laser illumination to form the interference fringes within the volume of a photosensitive medium. Upon development of the photosensitive medium, the pattern of the interference fringes is fixed within this medium in the form of varying indices of refraction. When light of certain wavelengths enters such a modulated index of refraction filter element, it is diffracted by the modulated index of refraction pattern therein.
In the case of laser production eyewear, it is common to form a modulated index of refraction filter element which reflects incoming radiation at the particular wavelength in a manner making it appear to be a mirror. Because the known laser generators employed in the combat environment includes a relatively limited number of wavelengths, it is possible to form a modulated index of refraction filter element for each of these wavelengths. The laser protective eyewear becomes, in effect, a reflection filter having a relatively narrow filter band about the expected wavelength of the laser source. It is possible to construct a compound structure including modulated index of refraction filter elements constructed to reflect differing wavelengths, in order to provide protection for a number of differing laser sources. Because the width of the spectral notch in such modulated index of refraction filter elements is relatively narrow normal visibility through such laser protective eyewear, even such eyewear having multiple modulated index of refraction filter elements for protection against a number of wavelengths, is relatively unimpaired.
Structures heretofore employed in such laser eye protection devices as goggles or visors do not solve the problems related to the provision of laser protection in a window. In particular, it is known in the art that such modulated index of refraction filter elements do not provide protection for all angles of incident radiation. These reflection modulated index of refraction filter elements provide a protection over only a cone of incident angles. Thus, the eye is not protected from laser illumination received at angles of incidence outside this cone. In the case of goggles or visors, it is possible to provide laser protection for greater angles of incidence employing position of the eye.
In accordance with U.S. Pat. No. 4,637,678, issued to Moss et el. on Jan. 20, 1987 entitled "Holographic Laser Protection Device," a compound holographic optical element structure is taught. A first holographic optical element covers angles of incidence about the normal to the surface of the visor, and a second holographic optical element covers angles of incidence oblique to the surface of the visor. In a second embodiment taught in that patent, the elements in a compound holographic optical element structure offer complementary coverage for angles of incidence from the right and from the left.
In accordance with U.S. Pat. No. 4,830,441 entitled "Holographic Filter Construction for Protective Eyewear," issued May 16, 1989 having the same assignee as the present invention, the geometry of the laser protection eyewear relative to the eye is exploited to provide greater angular coverage. This patent application teaches the use of geometries which are spherically symmetrical about the center of the eye rather than spherically symmetrical about the center of curvature of the protective element as previously taught.
The above teachings relative to goggles and visors cannot be applied to the case of a laser protection window. This is because the eye will not be at a fixed position relative to the laser protection goggles or visors. Therefore, there is a need in the art to provide some manner of laser eye protection through a window such as a vehicle window.